Organic electroluminescent (organic EL) is a light-emitting diode (LED) in which the emissive layer is a film made by organic compounds which emits light in response to an electric current. The emissive layer of organic compound is sandwiched between two electrodes. Organic EL is applied in flat panel displays due to their high illumination, low weight, ultra-thin profile, self-illumination without back light, low power consumption, wide viewing angle, high contrast, simple fabrication methods and rapid response time.
The first observation of electroluminescence in organic materials were in the early 1950s by Andre Bernanose and co-workers at the Nancy-University in France. Martin Pope and his co-workers at New York University first observed direct current (DC) electroluminescence on a single pure crystal of anthracene and on anthracene crystals doped with tetracene under vacuum in 1963.
The first diode device was reported by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1987. The device used a two-layer structure with separate hole transporting and electron transporting layers resulted in reduction in operating voltage and improvement of the efficiency, that led to the current era of organic EL research and device production.
Typically organic EL is composed of layers of organic materials situated between two electrodes, which include a hole transporting layer (HTL), an emitting layer (EML), an electron transporting layer (ETL). The basic mechanism of organic electroluminescence involves the injection of the carrier, transport, recombination of carriers and exciton formed to emit light. When an external voltage is applied to an organic light-emitting device, electrons and holes are injected from a cathode and an anode, respectively, electrons will be injected from a cathode into a LUMO (lowest unoccupied molecular orbital) and holes will be injected from an anode into a HOMO (highest occupied molecular orbital). When the electrons recombine with holes in the emitting layer, excitons are formed and then emit light. When luminescent molecules absorb energy to achieve an excited state, an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. 75% of the excitons form by recombination of electrons and holes to achieve a triplet excited state. Decay from triplet states is spin forbidden, Thus, a fluorescence electroluminescent device has only 25% internal quantum efficiency. In contrast to fluorescence electroluminescent device, phosphorescent organic light-emitting diodes make use of spin-orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and the internal quantum efficiency of electroluminescent devices from 25% to 100%.
Recently, a new type of fluorescent organic EL incorporating mechanism of thermally activated delayed fluorescence (TADF) has been developed by Adachi and coworkers is a promising way to obtain a high efficiency of exciton formation by converting spin-forbidden triplet excitons up to the siglet level by the mechanism of reverse intersystem crossing (RISC.
The phosphorescent organic EL utilizes both triplet and singlet excitons. Cause of longer lifetime and the diffusion length of triplet excitons compared to those of singlet excitons, the phosphorescent organic EL generally need an additional hole blocking layer (HBL) between the emitting layer (EML) and the electron-transporting layer (ETL) or the electron-transporting layer with hole blocking ability instead of typical ETL. The purpose of the use of HBL or HBETL is to confine the recombination of injected holes and electrons and the relaxation of created excitons within the EML, hence the device's efficiency can be improved. To meet such roles, the hole blocking materials must have HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy levels suitable to block hole transport from the EML to the ETL and to pass electrons from the ETL to the EML, in addition, the good thermal and electrochemical stability of the materials are also needed.
Currently, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), bathophenanthroline (Bphen) have been used as the typical materials for the HBL and HBETL of phosphorescent OLED. However, phenanthroline derivatives exhibit lower Tg (Bphen: 55° C., BCP: 65° C.), lower heat-resistant (Td: Weight loss <0.5% at 240° C. for Bphen and 260° C. for BCP). It's difficult to operate under deposition process and its devices show lower stability and short half-life time. U.S. Pat. No. 7,119,204 disclose a series of substituted-phenanthroline derivatives, as electron-transporting materials. U.S. Pat. No. 7,282,586 disclose a specific phenanthroline derivative 2,9-bis(5-(biphenyl-4-yl)-1,3,4-oxadiazol-2-yl)-1,10-phenanthroline, as an electron transporting material, compare with conventional 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), the drive voltage is decreased from 8V to 7V at 2000 cd/m2, and higher current efficiency is achieved. U.S. Pat. No. 7,754,348 disclose a series of 2,9-substituted phenanthroline derivatives as electron transporting material, higher operate live time and higher luminance than comparable example 1-3 and Alq3 has also been achieved at a driving voltage of 5V. U.S. Pat. No. 7,982,213 disclose a series of aryl substituted phenanthroline, a phosphorescent organic EL using the phenanthroline compound as HBETL provided high efficiency and a high luminance and has a high long-term durability. U.S. Pat. No. 8,114,529 disclose a series of bis-phenanthroline skeleton compounds, by using the phenanthroline compounds as HBETL, phosphorescent organic EL having low driving voltage and excellent durability.
There continues to be a need for organic EL materials which is able to efficiently transport electrons and block holes, with good thermal stability and high emitting efficiency. According to the reasons described above, the present invention has the objective of resolving such problems of the prior-art and offering a light emitting device which is excellent in its thermal stability, high luminance efficiency, high luminance and long half-life time. The present invention disclose a novel phenanthroline-based compound having general formula (I), used as hole blocking material/electron transport material or phosphorescent host have good charge carrier mobility and excellent operational durability can lower driving voltage and power consumption, increasing efficiency and half-life time of organic EL device.